Note: A common clay mineral of the kaolin group; hydrated aluminum silicate mineral which is the principal constituent of kaolin clay. Kaolinite crystals have a lamellar (plate-like) structure that gives the clay its slippery feel. It is a natural alteration product of aluminum silicate rocks, such as feldspar. Pure kaolinite is a primary clay; most secondary clays contain only a small percentage of kaolinite, although an exception is ball clay found in Wareham, England.